1. Field of the Invention
The present invention relates to an active-matrix liquid crystal display (AM LCD) device, and more particularly, to a field sequential liquid crystal display device and a method for displaying color images using the field sequential liquid crystal display device.
2. Discussion of the Related Art
Until now, the cathode-ray tube (CRT) has been generally used for display systems. However, flat panel displays are increasingly beginning to be used because of their small depth dimensions, desirably low weight, and low power consumption requirements. Presently, thin film transistor-liquid-crystal displays (TFT-LCDs) have been developed with a high resolution and small depth dimensions.
Generally, a liquid crystal display (LCD) device includes an upper substrate, a lower substrate, and a liquid crystal layer interposed therebetween. The upper and lower substrates respectively have electrodes opposing to each other. When an electric field is applied between the electrodes of the upper and lower substrates, molecules of the liquid crystal are aligned according to the electric field. By controlling the electric field, the liquid crystal display device provides various transmittances of incident light to display images.
In these days, an active-matrix liquid crystal display (AM LCD) device is the most popular because of its high resolution and superiority in displaying moving images. A typical active-matrix liquid crystal display has a plurality of switching elements and pixel electrodes, which are arranged in an array matrix on the lower substrate. Therefore, the lower substrate of the active-matrix liquid crystal display is alternatively referred to as an array substrate.
The structure of a conventional active-matrix liquid crystal display will be described hereinafter with reference to FIG. 1, which illustrates a cross section of a pixel region. The liquid crystal display 10 consists of a liquid crystal panel 15 and back light 50. The liquid crystal panel 15 includes a color filter substrate (i.e., an upper substrate) 20 and an array substrate (i.e., a lower substrate) 40 which face each other across a liquid crystal layer 30. Within the color filter substrate 20, a color filter consisting of red (R), green (G), and blue (B) and a black matrix 22b are formed on a transparent substrate 1 for preventing a light leakage. The common electrode 24, which functions as one electrode for applying a voltage to the liquid crystal layer 30, is formed on the color filter 22a and black matrix 22b. 
Within the lower substrate 40 of FIG. 1, a thin film transistor “T” functioning as a switching element is formed over the transparent substrate 1 facing the upper substrate 20. A pixel electrode 42, which is electrically connected to the thin film transistor “T” and serves as another electrode for applying a voltage to the liquid crystal layer 30, is formed over the transparent substrate 1 of the array substrate 40. The back light 50 is disposed under the array substrate 40 to irradiate light to the liquid crystal panel 15. Although not shown in FIG. 1, the thin film transistor generally comprises a gate electrode, a source electrode and a drain electrode.
This liquid crystal display device described above uses an optical anisotropy and polarization property of liquid crystal molecules for displaying a desired image. That is, applying a voltage to the liquid crystal molecules having a thin and long structure and pretilt angle changes an alignment direction of the liquid crystal molecules. Thereafter, light incident from the back light device is polarized due to the optical anisotropy of the liquid crystal molecules. And lastly, the polarized light is modulated by passing through the color filter layer, and thus color images are displayed.
But the conventional active-matrix liquid crystal display device has some problems as follows. Firstly, the material used for the color filter is expensive, resulting in an increase of the manufacturing cost. Secondly, because the transmissivity of a material used for the color filter is less than 33% so that a brighter back light is required in order to display a color image effectively, which results in the increase of the power consumption.
Research and development have been conducted recently in an effort to overcome these problems. Therefore, a field sequential liquid crystal display (FS LCD) device, which displays a full color without the color filters, is suggested as an alternative.
The conventional active-matrix liquid crystal display devices display the color image by constantly transmitting white light from the back light to the liquid crystal panel, whereas the FS LCD devices display the color image by sequentially and periodically turning on and off the light sources having Red, Green and Blue colors. Though the field sequential liquid crystal display device has not been popular until recently because of the lack of a short response time of the liquid crystal molecules, it can be popularized in the field thanks to a development of new liquid crystal molecules having a short response time, such as Ferroelectric Liquid Crystal (FLC), Optical Compensated Birefringent (OCB) and Twisted Nematic (TN).
In addition, the Optical Compensated Birefringent (OCB) mode is generally used for the field sequential liquid crystal display device because the OCB mode forms a bend-structure and the response time thereof is less than about 5 msec when the voltage is applied thereto. Therefore, the OCB mode liquid crystal cells of the OCB mode are suitable for the field sequential liquid crystal display device owing to the short response time leaving no residual image on a screen.
FIG. 2 is a cross-sectional view illustrating the schematic cross section of the conventional field sequential liquid crystal display device. The conventional field sequential liquid crystal display device 60 includes an upper substrate 64 (referred to as a color filter substrate), a lower substrate 66 (referred to as an array substrate), a liquid crystal layer 70 interposed therebetween and a back light device 72 consisting of three light sources Red (R), Green (G) and Blue (B) to irradiate light to the liquid crystal panel 62. A black matrix 61 is formed between the common electrode 65 and the transparent substrate 1 of the upper substrate 64 in order to prevent leakage of light in a non-display region other than a region for a pixel electrode 67. A thin film transistor “T”, which functions as a switching element and is electrically connected to the pixel electrode 67, is formed over the transparent substrate 1 of the lower substrate 66. The thin film transistor “T” corresponding to the black matrix 61 consists of gate, source and drain electrodes (not shown).
The biggest difference of the field sequential liquid crystal display (FS LCD) device 60 with the conventional liquid crystal display of FIG. 1 is that the FS LCD device does not need the color filters in the upper substrate 64 and has a back light device that includes three different light sources that are sequentially and selectively turned on and/or off. The light sources having Red (R), Green (G) and Blue (B) colors are driven respectively by an inverter (not shown) and each of Red, Green and Blue light sources is turned on and off sixty times per second, resulting in one hundred and eighty times per second in all.
Therefore, a color image caused by the mixture of three colors (red, green and blue) is displayed using an afterimage (i.e., residual image) effect of human vision. Though the Red, Green and Blue light sources are turned on and off one hundred and eighty time per second, the perception by the naked eye is that the light sources are kept on due to the afterimage (or residual image) effect. For example, if the Red light source is turned on and then the Blue light source is sequentially turned on, a mixed color (i.e., violet) is shown owing to the residual image effect.
Since the FS LCD devices do not need the color filters, the FS LCD devices overcome the problem that the conventional active-matrix liquid crystal display devices cause the decrease of the luminance due to the color filters. In addition, the FS LCD devices are suitable for the liquid crystal display devices of a large scale because they can display a full-color using three-color light sources whereby they can display an image of high luminance and high resolution. Though the conventional active-matrix liquid crystal display device is inferior to CRT (Cathode Ray Tube) in terms of price or resolution, the field sequential liquid crystal display device can solve these problems.
FIG. 3 is a flow chart schematically showing an operation of a field sequential liquid crystal display device according to a conventional color image display method. In the initial step “st1”, a single frame as an image display unit is divided into three subframes each having one—one hundred eightieth of a second ( 1/180 second) period. In step “st2”, electric signals are applied to pixels of the FS LCD panel at 1/180 second interval. At this time when the electric signals are applied, the thin film transistors are operated as switching devices such that the liquid crystal molecules are arranged according to the signals. Further within one frame, the primarily arranged liquid crystal molecules of one pixel continue to maintain their status until the liquid crystal molecules of the last pixel are arranged. In step “st3”, when the liquid crystal molecules of the designated frame are all arranged, the light sources are turned on in the designated pixel. Namely, the light sources of the backlight device of the conventional FS LCD device are turned on sequentially, respectively, periodically and repeatedly without the additional control devices.
FIG. 4 is a graph showing a gray level of the emitted light depending on a light source. In general, the liquid crystal panel for the FS LCD device does not include the color filter contrary to the conventional LCD device, such that the liquid crystal panel displays a black color unless the light source irradiates light. The gray level of the initially inputted signal is defined by multiplying a gray level of the black-and-white liquid crystal panel by a gray level of backlight. As shown in FIG. 4, the Red, Green and Blue light sources forms one frame “1f” and are sequentially turned on/off. The brightness of Red, Green and Blue light sources is respectively represented by L1, L2 and L3 in FIG. 4. In this graph of FIG. 4, if the gray level of inputted signal and the gray level of black liquid crystal level are maintained at fixed values, it is obvious the picture brightness depends on the backlight.
However, since the Red, Green and Blue light sources are sequentially turned on and off in the conventional FS LCD devices without extra control devices, the maximum brightness is limited to 1b that represents the brightness L2. Namely, when the brightness L2 of the Green light source is calculated in gray level (i.e., 1b), the gray level 1b represents the maximum brightness among the light sources such that the maximum brightness of the Red, Green and Blue light sources is less than the gray level 1b. 
FIG. 5 is a graph of the lighting time of the subframes, plotted as a function of the time according to the Red (R), Green (G) and Blue (B) light sources. As shown in FIG. 5, 1/60 second as one frame (1f) is divided into first sf1, second sf2 and third sf3 subframes. At this time, each Red (R), Green (G) or Blue (B) light source of the subframes is substantially turned on for less than 1/180 second because the duration of each subframe sf1, sf2 or sf3 takes into account the duration of applying the electric signal, aligning the liquid crystal molecules and turning on the backlight device. Therefore, if each light source of the subframe is thoroughly turned on for 1/180 second, the light leakage can occur because the light is irradiated before the aligning of the liquid crystal molecules. Furthermore, the color interference may occur between the light sources of the subframes. In other words, switching on and off the light source of each subframe is carried out after applying the electric signals and aligning the liquid crystal molecules, and depends on the thin film transistors and the condition of the liquid crystal molecules.
However, since the conventional FS LCD devices does not have a control device controlling the light sources of the backlight device, the light leakage and the decrease of display quality occur in the conventional FS LCD devices whenever the design of the thin film transistor changes.